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The ice of Antarctica Almost 90% of the planet's fresh , is locked up in the and ice of Antarctica. There is so much ice it covers an area twice that of Australia and produces an that is the single largest object on the surface of Earth - so heavy it deforms the crust beneath. This huge volume of ice is not constant, but grows during ice ages and shrinks between them. Every year new snow accumulates on the surface of the continent, but due to the persistent cold does not melt, instead it is compressed by the weight of new snow and becomes ice. This ice preserves a rich and detailed record The ice cap of Antarctica is the largest solid object on Earth. spanning more than a million years, giving us a detailed Image Stephen Hudson, Wikicommons view of Earth’s history, even before modern humans walked its surface. Ice cores From the centre of Antarctica ‘glacial conveyor belts’ By drilling into buried ice and extracting air samples, transport 200 billion tonnes of ice each year, to ice shelves scientists can build a record of past . Some of at the edge of the continent. These ice shelves then travel these cores are over three kilometres deep, and provide a across the surface of the ocean before breaking apart to continuous Antarctic record dating back 800,000 years - create a flotilla of which drift northwards into the allowing us to better understand how the current rate of currents of the Southern Ocean. global warming compares to past events and cycles. In this way the volume of fresh water released from One chemical tool used to examine ice cores counts Antarctica is a delicate balance between snowfall and the the different forms (isotopes) of oxygen and hydrogen. loss of ice to the Southern Ocean - a process that plays a Since the ratio of these isotopes is determined by air major role in controlling the world’s climate, as well as all temperature, the trapped air records the temperature the its physical and biological systems. day the snow fell. By making numerous measurements climate changes 'mass balance' can be found, such as that showing how Earth naturally experiences cycles of increasing and decreasing How much ice is gained or lost over time is known as greenhouse gases. This data also indicates that today's an ice sheet's mass balance. If there is no gain nor loss greenhouse gas concentrations are the highest in 420,000 the ice sheet is said to be in equilibrium. years - possibly through the burning of fossil fuels. The mass balance of a single can be estimated but trying to do this for a whole continent is very difficult, as each glacier may behave differently. However since the PROXIES 1960's man-made satellites have allowed the height, Proxies are clues or indirect evidence about an event. temperature and flow rates of large areas of ice to be They are especially useful when real evidence, such as measured - with some satellites now accurate to the temperature, cannot be preserved. Using proxies enables nearest centimetre. Antarctic scientists to: • identify seasonal and annual snowfalls. Snow and ice formation • get accurate dates or ‘time stamps’ by chemically Snow is formed when moisture evaporates from open examining layers of volcanic ash. water, cools and condenses on tiny particles such as dust. • date layers by measuring levels of radioactivity. These frozen particles gather more water around them and • estimate ancient ice cover, photosynthesis and form snowflakes, that eventually fall and settle. pollution by measuring trace elements and dust levels. Since moisture and cold are essential for snow, coastal areas are regularly battered by snow , which can Dust and climate bring more than two metres of snow per year.These Dust levels in an are an important clue about coastal storms rarely penetrate inland - instead tiny past climates. This is because during cold (glacial) times, granules of ice, known as drift down through more fresh water is locked up in ice caps and , so the atmosphere and settle on the surface of the ice sheet. that the remaining land is drier and desert like. At such Each granule of diamond dust is so much smaller than a times winds also tend to be stronger, due to the greater snow flake that at the South Pole only 2 - 5cm of snow temperature difference between the equator and the poles, accumulates each year - equivalent to the rainfall of the so more dust appears in the ice core records. During Sahara desert. warmer periods the opposite happens and the ice is often • As snow is compressed air is expelled, making the cleaner. snow flakes rounder and forming firn snow. • As firn is compressed the crystals are compressed further sealing air into individual pockets or pores and forming bubbly snow. This air makes up a tenth of the volume of bubbly snow and is an important sample of the atmosphere on the day the snow fell.

For a full range of Antarctic and Southern Ocean resources visit: The Antarctic Hub www.antarctichub.org Antarctic Drainage

Ice flows outwards from the centre of Antarctica from nearly 4000m above sea level in the middle of the East Antarctic Ice Sheet - rather than from the South Pole as many imagine. Despite the scale and speed of its movement the flow is actually controlled by the underlying rock formations and at a microscopic level by the way individual slide past each other. The ice moves fastest at the surface, and more slowly near the bedrock due to friction and shearing. For this reason, horizontal layers in the ice sheet move past each other at different rates - rates which vary from a few metres to several hundred metres a year. The Peltier Channel. Recent spectacular collapses of ice shelves There are five major drainage systems in Antarctica, around the AP, may cause a ‘runaway’ release of ice from the with each made up of ice streams, glaciers and sub-glacial continent. Image: Serge Ouachée, Wikicommons water flow. These systems mostly flows into the ice shelves which fringe the continent, although some glaciers glaciers which drape the valley walls and terminate in flow directly into the sea. near-vertical ice cliffs. This spectacular appearance is made possible because they lack water at their base, but 1. Ice streams instead are frozen to the bedrock. As a consequence Dry Ice streams are giant rivers of ice and the first were Valley glaciers move very slowly, contain very old ice and discovered only when satellite images of Earth became are rich in sediment at their base. available in 1972. These images showed ice streams cutting through the Antarctic ice sheet and being fringed by Antarctica Peninsula glaciers wide zones of crevasses - a feature which indicated they The shape and action of glaciers on the Antarctic were flowing ten to a hundred times faster than the Peninsula are controlled by the mountains down which surrounding ice. Like a river, ice streams flow fastest in the they flow, making them more like glaciers on other centre and more slowly at the edges. These fast flow rates continents, complete with ice filled valleys and cirques. were difficult to explain so glaciologists drilled bore holes Most Antarctic Peninsula glaciers have been retreating into the ice, sending cameras and instruments to the rapidly in recent decades, in response to rising bedrock below. Here they found a water-filled, glacial temperatures in the area. sediment called till, which lubricated the ice stream's movement. Despite the cold, water flows beneath the ice 3. Ice shelves stream due to pressure melting, which can also be seen if Ice shelves are floating rafts of ice, formed when you squeeze a snowball in your hand. glaciers reach the coast. Almost half of Antarctica. is The conditions beneath the ice are important in fringed by ice shelves, which although appearing flat, controlling ice streams and therefore the whole stability of featureless and unmoving, are often fast moving, reaching Antarctica, especially since ice streams stop and start very speeds of several kilometres a year. quickly. Of special concern are the ice streams of the The largest ice shelves are the Ross, the Ronne- Amundsen Sea Coast (West Antarctica), as they are Filchner and the Amery, but there are also many other strongly out of balance with the rest of the ice sheet, smaller ice shelves around the coast of Antarctica. All are resulting in ice accelerating, thinning and showing rapid fed by the flow of inland ice, but also gain ice by seawater loss across some of the largest glaciers in the region. freezing on to their base and from snowfall above. Ice Considering these glaciers have grounding lines close to shelves loose mass mainly by shedding icebergs off their the ocean they are particularly vulnerable to rapid melting. seaward edge into the ocean, in a process known as The discovery of ice streams and understanding their calving, which creates vertical cliffs of ice. Around the behaviour has been one of the greatest glaciological Ross Ice Shelf calving has produced impenetrable cliffs advances in the history of Antarctic science. that rise 50 metres from the sea, which prompted early explorers to name it ‘The Barrier’. 2. Glaciers Ice shelves flow from the land and slide over the sea There are three main types of Antarctic glacier. floor until the water is deep enough to allow them to float. Outlet Glaciers The line where an ice shelf floats, no longer touching the Unlike ice streams, which cut a path through an ice sea floor, is called the grounding line. The thickest ice is sheet, outlet glaciers flow through mountain ranges generally found at the grounding line, as the ice thins as it following the shape of the valleys beneath. The outlet spreads out over the ocean. With a warming climate only glaciers of Antarctica provided routes which the early ice inland of the grounding line will contribute to sea level explorers used to cross the Trans Antarctic Mountains as rise, as ice that is seaward of the grounding line is already they headed to the South Pole. floating. The position of the grounding line is very sensitive to climate change; Dry Valley glaciers • in cold climates when the ice sheet expands, the The 'Dry Valleys' of Antarctica are so named because grounding line will move further into the ocean. they are ice free. Although such glaciers are rare in • in warming climates the grounding line retreats Antarctica they create of its most striking features, for instance the McMurdo Dry Valleys have numerous such For a full range of Antarctic and Southern Ocean resources visit: The Antarctic Hub www.antarctichub.org 4. Icebergs Icebergs are fragments of ice shelves that form when the ice breaks along lines of weakness, caused by ocean currents and waves. As ice bergs are pushed by wind and currents they all travel west around the continent before moving north to be caught in the easterly flow (West Wind Drift). By the time icebergs have moved far enough north to reach the Antarctic Convergence they have usually disintegrated, but on rare occasions icebergs have travelled as far north as 35 degrees into the Indian and South Atlantic Oceans. Some of the largest icebergs known have originated from the ice shelves of the Ross Sea. The largest recorded is B-15, which calved off the Ross Ice Shelf in 2000 and When the congeal into frozen , the size of the Antarctic had an area of over 11,000 km² (larger than Jamaica). This almost doubles and the continent becomes almost inaccessible. giant prevented ocean currents and winds from breaking Image: Ville Miettinen, Wikicommons. up the sea ice, and even caused a decline in 2.First year sea ice is initially salty but with time the penguin populations due to the extra distances that salty brine drains down through the ice. If this ice is parents had to cover to reach their chicks from open water. frozen to the continent, it is known as fast ice, but if it rests on open ocean it is known as pack ice 5. Sea ice 3. Multi-year ice is ice that has survived more than one In , the area covered by ice in Antarctica winter so is the least salty. It is also the least dense approximately doubles, as the surface of the ocean because of the empty holes (brine pockets) near its freezes. This extra ice extends up to 1000 km from the surface. coast and covers an area of 20 million square kilometres. The annual growth and loss of Antarctic sea ice follows a seasonal pattern: 6. Ice floes • during May and June, the sea ice front advances by Sea ice is not a single uniform sheet of ice, but is up to 4 km a day. As it reaches outward it also grouped into floes. These floes move with wind and thickens due to snowfall and freezing of the seawater currents, and may join together forming 'pressure ridges' beneath. where they meet.The areas of open water between floes • during September - October the extent of sea ice is at are known as polynyas. Polynyas usually last a long time a maximum, reaching 3-4 metres. and occur in the same place, due to ocean currents and • by the end of February Antarctic sea ice has melted water upwelling from below. These islands of open water back to the edge of the continent. are crucial for some organisms to survive the cold winter months. The seasonal effects of sea ice The change from dark open ocean to solid, white ice (and back) has enormous effects on Antarctica - for example: • in winter the sea ice acts as a ‘blanket’ over the ocean and prevents the evaporation of moisture into the atmosphere. This is one of the main reasons Antarctica is so dry. • in , when the sun returns to Antarctica the brightness of the sea ice reflects much of the sun's energy into the atmosphere, slowing the rate at which the continent warms. • in summer, when the sea ice has retreated, the atmosphere around Antarctica is more humid, and coastal regions receive more snow. The increase in open water also allows more light to penetrate into the sea, which increases the rate of photosynthesis and allows food chains to flourish.

Types of Sea Ice There are many different types of ice in Antarctica. Sea ice varies but there are three important forms: 1. is the first to form and begins as small needles, these then become a delicate layer of tiny ice plates. As frazil ice thickens it forms an icy sludge known as grease ice. If conditions are calm grease ice can grow rapidly into solid sheets called nilas, but in stormy seas it may be rammed together into pancake Adapted from material by Kate SInclair, GNS. ice. If thickens further it forms floes, which by Donald Reid, iMatters.co.nz in association with Gateway eventually grow into first year sea ice. Antarctica, University of Canterbury. See following page for practical ideas. Practical activity: Making sea ice Introduction Sea ice forms differently from other forms of ice mainly because it: • cools by loosing heat from the upper surface, into the atmosphere • thickens by the continual formation of new ice on the underside of sea ice • contains salt, which is lost from beneath the ice as it freezes

What to do 1. Cut the top of a large bottle. 2. Add 1 litre of tap water and 35 grams (2 Tps) of table salt. 3. Stir until all the salt is dissolved. 4. Wrap the bottle's sides and base with an insulating layer (eg old towels, clothing, newspaper), but leave the top surface of the salty water exposed. 5. Place the bottle and insulation in a freezer, for about two to four hours.

How it works Image: iMatters As the water in the sea freezes it forms flat sheets of platelet ice, which float upwards. As cooling continues the platelets join to form a solid mass of ice. As the water freezes the salt is excluded and sinks downwards.

Note: The ice becomes less salty, while the solution (sea) below becomes more salty. This can be easily be tested by tasting the saltiness of a small sample of these three • the salty water at the start • the water from beneath the ice. • the ice

Relevance • Sea ice forms due to loss of heat from the sea into the colder atmosphere above. • Sea ice forms on the sea surface, insulating the water below and preventing whole oceans freezing. • Salt sinking beneath sea ice helps to drive the circulation of ocean currents around the world.

Practical activity: Make an ice flow model For details visit : www.gns.cri.nz/Home/Learning/Science-Topics/Ice-Snow/Lesson-Plans

Practical activity: Make an model of Antarctic ice Using the materials below, make a model which shows features in the diagram. • plastic container • rocks (as bedrock) • sediment (sand or gravel) • fresh and salty water

Note: Your model will have to be made in stages, rather than in one step.

For a full range of Antarctic and Southern Ocean resources visit: The Antarctic Hub www.antarctichub.org